This invention relates to burning powdered coal.
The advantages of coal as an energy source need no elaboration here, at this historical juncture. However, burning coal has also had its disadvantages: initial handling, handling of waste products of combustion, equipment corrosion, and pollution.
In recent years, fluidized bed combustors have been developed that solve most of these problems. In particular, desulfurization of high-sulfur coals by limestone addition to the combustor is done more reliably and cheaply in fluid bed combustors than by stack-gas scrubbers, which are still unproven. Particulates may also be removed more cheaply from fluidized bed combustors than from conventional coal burners, by substituting low-cost cyclone separators for expensive electrostatic precipitators. Cyclones can be used with fluid beds because of the ash-coarsening agglomeration that occurs, as is shown in Godel U.S. Pat. No. 3,431,892 and Stephens et al. U.S. Pat. No. 3,171,369. The emission of noxious trace elements, (e.g., beryllium, cadmium, and mercury) is minimized in fluidized beds by their far-lower operating temperature.
Yet, the low outlet temperatures of conventional fluidized beds outlet gases cannot be used to provide heat for high-temperature furnaces. Such low temperatures also create excessive stack-gas losses if conventional fluidized-bed combustors are retrofitted to existing boilers and space-heating furnaces. The present invention overcomes these objections by providing a system with high outlet temperatures that nevertheless permits the char combustor to be operated at low temperatures. One prior art system that also produces outlet temperatures appreciably higher than the char-burner temperature is found in Way U.S. Pat. No. 3,358,624. The system uses a pyrolyzer to separate coal into char and volatiles, a burner for the char cooled by excess air, but only to keep the ash molten so that it will form as slag on the walls rather than be vaporized, and a second burner for the volatiles and combustion products and excess air of the first burner, giving a high output temperature.
Numerous pyrolyzers have been developed over the years to remove volatiles from coal. One requirement is for a pyrolyzer that can handle caking coals without fouling. The design perfected by the U.S. Bureau of Mines (Bureau of Mines Report of Investigation 7843, 1973), is particularly suitable for the needs of the present invention. The design has been modified to withdraw solid materials in a standpipe, instead of by gas entrainment from the top, thereby reducing particle attrition; this modification is suggested in "Spouted Beds," Kishan B. Mathur, Norman Epstein, Academic Press, New York, 1974.
Another problem with conventional fluidized bed combustors relates to the large freeboard volume which is required to assure the complete burnout of burning particles. The large space contributes significantly to the burner size and cost, and can be reduced by introducing the still-burning particles into a swirling space. This technique has been taught in oil burners in Cox, Editor, "Gas Turbines Principles and Practices." (George Newes, Ltd., London, 1955, p. 17-14 and -15).